Intel® PXA255 Processor Developer’s Manual 8-5
Synchronous Serial Port Controller
Figure 8-2 shows one of the four configurations for the Motorola SPI frame format for single and
back-to-back frame transmissions.
8.4.1.3 Microwire Format Details
Microwire format is similar to SPI, but it uses half-duplex trans missions with master-slave
message passing rather than full-duplex. In idle state or when the SSP is disabled, SSPSCLK is
low, SSPSFRM is high, and SSPTXD is low.
Each Microwire transmission begins with SS PSFRM assertion (low), followed by an 8 or 16-bit
command word sent from controller to peripheral on SSPTXD. The command word data size is
selected by the Microwire Transmit Data Size (MWDS) b it in SSP Control Register 1. SSPRXD is
controlled by the peripheral and remains tristat e d. SSPSCLK goes high midway through the
command’s most significant bit and continues to toggle at the bit rate.
One bit-period after the last command bit, the peripheral must ret urn th e serial d ata r equested , most
significant bit first, on SSPRXD. Data transitions on SS PS C L K’s falling edge and is sampled on
the rising edge. SSPSCLK’s last falli ng edge coi ncides wi th the e nd o f the las t d ata bit on SSPR XD
and it remains low if it is the only or last word of the transfer. SSPSFRM deasserts high one-half
clock period later.
The start and end of a series of back-to-back transfers are similar to those of a single transfer.
However, SSPSFRM remains asserted (low) throughout the transfer. The end of a data word on
SSPRXD is immediately followed by the start of the next command byte on SS PTXD.
Figure 8-2. Motorola SPI* Frame Format
SSPSCLK ...
SSPSFRM ...
SSPTXD Bit<N> Bit<N-
1> ... Bit<1> Bit<0>
SSPRXD Bit<N> Bit<N-
1> ... Bit<1> Bit<0>
MSB 4 to 16 Bits LSB
Single Transfer
SSPSCLK ... ...
SSPSFRM ... ...
SSPTX /RX Bit<0> Bit<N> Bit<N-
1> ... Bit<1> Bit<0> Bit<N> Bit<N-
1> ... Bit<1> Bit<0>
Continuous Transfers
Note: SSPSCLK’s phase and polarity can be configured for four modes. This example shows one of those modes.